This invention relates to well cementing.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, a conduit (casing) must be inserted into the borehole extending from the surface downward, and liners may be hung inside the casing.
At this point it becomes necessary to dispose of the used drilling fluid and to fill the annulus between the casing and the borehole wall or between the liner and casing with a material which will seal the annulus (interfacial sealing) to inhibit communication between various formations penetrated by the wellbore and which will provide structural support for the casing or liner. This is commonly referred to as primary cementing.
Bonding of the cement to the casing and borehole surfaces is critical to providing an effective seal in the annulus and for providing support for casings. However, bonding or interfacial sealing between the cement and borehole surfaces is not readily achievable. Generally, the borehole into which the casing or liner is introduced is filled with drilling mud. Conventional Portland cement and conventional drilling muds are incompatible. Thus, a mixture of conventional Portland cement and conventional drilling mud will not set up into a strong cement. In addition, the viscosity of such mixtures becomes uncontrollable and may either become too viscous to pump or may get thinner.
At the completion of drilling, the used drilling fluid is displaced from the borehole using some means to keep it separate from the cement to follow. This creates two problems. First, the means developed by the industry to keep the drilling fluid separate is relatively complex, involving the use of a landing collar and a pair of wiper plugs. In addition, the thus-displaced drilling fluid must be disposed of. Wyant et al, U.S. Pat. No. 3,499,491 (Mar. 10, 1970) proposed a partial solution to this problem by mixing a cementitious material such as Portland cement with powdered sodium silicate glass and a treated drilling fluid. While this does solve the problem of drilling fluid disposal since the drilling fluid is incorporated into the cement, it necessitates the use of extraneous components in order to achieve a sufficient degree of compatibility to make the cement work at all.
Over the years there has been a tendency for wells to be drilled to greater depths and in exotic locations as the supply of easily obtained oil is diminished. This has resulted in the necessity to drill and cement under conditions of greatly elevated temperatures. Portland cement is particularly sensitive to temperature variations and thus conventional cementing of high temperature wells poses a significant problem to the industry.
Hale and Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991) disclose solidifying drilling fluid by admixing blast furnace slag with the drilling fluid. Tragesser, U.S. Pat. No. 3,557,876 (Jan, 26, 1971) refers to drilling fluids containing pozzolans although no working examples are given.
It would be desirable to have a low cost cementitious material which could be solidified over a wide range of temperature including the very high temperatures now encountered in many drilling operations and which woould give good interfacial sealing.